In furnaces used throughout the metallurgical and related industries to heat metallic workpieces such as slabs, blooms, billets or other raw steel shapes prior to further processing into useful finished products, the typical furnace includes a complex network of insulated vertical and horizontal water-cooled pipes which support an additional network of water-cooled skid pipes with rails along which the slabs, blooms, billets or other raw steel shapes are pushed or walked through the furnace. The metallurgical furnace is an open system; that is, furnace heat which is lost to the metal pipe network is then conducted by the flowing water to a point outside the furnace and is not recoverable. Accordingly, vast amounts of heat losses occur and correspondingly unnecessary amounts of energy are expended to replace the heat loss transferred into the water-cooled pipe network. For example, in a furnace having a water-cooled piping infrastructure of uninsulated skid pipes and supporting pipe, as much as twenty-five percent of the total heat supplied to a metallurgical furnace by the combustion of fuels may be lost to the water-cooled skid pipe infrastructure. For a 6 inch OD uninsulated water-cooled pipe in a furnace operating at 1800.degree.- 2400.degree. F., the heat loss is approximately 60,000-140,000 Btu per lineal foot per hour. For a furnace having 600 feet of skid pipe, 200 feet of horizontal support pipe and 400 feet of vertical support pipe, the heat loss is thus approximately 1200.times.100,000 Btu/hour or 120,000,000 Btu/hour.
In order to reduce the heat loss, various types of refractory and insulating materials have been affixed to the outside surface of the water-cooled piping. The variety of attachment methods and materials has included the use of pre-fired or chemically bonded refractory materials which are welded, studded, wired, clipped or anchored with interlocking devices. Moreover, refractory concrete materials have even been formed in place around the pipe surfaces, the refractory concrete being supported by a great number of metallic anchors which are individually welded to the pipe surface. Almost without exception, these forms of insulation have failed within a relatively short period of operation because of the inherent friability and susceptibility to fracture of the brittle concrete, fired ceramic or chemically bonded refractory materials. As the raw steel workpiece is moved along the metal skid rail pipe, significant vibration and flexion of the water-cooled pipe infrastructure occur which are in turn transmitted into the insulation material. Consequently, the combination of high temperature furnace gases and the flexion and vibration within the pipes fractures the brittle refractory material and oxidizes the metallic anchors, thereby causing early failure of the insulation which falls to the furnace floor.
Water-cooled skid pipe infrastructures having complex cross-sectional geometry of the individual pipe elements to achieve greater mechanical strength are currently in wide use in metallurgical heating furnaces. The greater strength of these pipes allow a significant reduction in the total number of support elements in the skid pipe infrastructure and a consequent reduction in the potential heat loss to the water-cooled skid pipes. These skid pipes have complex cross-sectional geometry and often vary in dimension due to deviations in manufacturing or fabrication methods. These variations cause undesirable gaps between the outer surface of the pipe and the inside surface of the pre-formed insulation shapes. The presence of these gaps results in premature insulator failure caused by over-heating and oxidation of the internal insulator metallic anchors which, in order to provide an effective and efficient conductive cooling path to prevent overheating and oxidation, must always be in very close and intimate communication with the water-cooled skid pipe.
A current method often used to insulate water-cooled skid pipes of complex cross-sectional geometry incorporates the use of (1) individually positioned metallic anchors welded directly to the outer surface of the water-cooled ski pipe; (2) a resilient ceramic fiber insulating blanket wrapped around the water-cooled skid pipe; and (3) an outer layer of durable refractory concrete. The anchors are usually installed in a pattern that results in a uniformly spaced apart distance of 3-4 inches, resulting in a requirement for as many as 75,000-90,000 individually welded anchors for a typical water-cooled skid infrastructure. The layer of ceramic fiber insulating blanket is wrapped around the skid pipe, impaled upon the welded metallic anchors, and compressed to result in protrusion and exposure of 60-70 percent of the metallic anchors length. A variety of appropriate forms are then positioned around the ceramic fiber to allow for casting and shaping of an outer refractory concrete layer of the insulation system. After proper curing of the refractory concrete, the forms are removed and the refractory concrete is further allowed to dry. This method of affixing insulation to the skid pipe surface is an improvement over the older method of casting refractory concrete directly in contact with the water-cooled skid pipe since the intermediate layer of resilient ceramic fiber blanket acts as a cushion that absorbs some of the pipe vibrations and reduces the effect of skid pipe flexion on the insulation.
Those skilled in the art understand that the periodic replacement of this type insulation is required because of other furnace operating factors such as chemical attack by hot furnace gases, fluxing agents, products of combustion, and the effect of periodic thermal cycling of the furnace.
Those skilled in the art also understand that the replacement of this welded metallic anchor/ceramic fiber/refractory concrete insulation is complex and time consuming requiring the complete removal of the oxidized metallic anchor remains by scarfing and refurbishment of the scarf damaged original pipe surface. Experience has shown that the total time for re-insulation often takes 14-21 days depending on the total lineal feet of water-cooled pipe network within the furnace. This furnace "downtime" is costly to the operator.